a. Field of Invention
The invention relates generally to prosthesis devices, and, more particularly to an externally powered hand prosthesis mechanism for use with persons with amputations at or proximal to the level of the metacarpophalangeal joint, as well as persons with high-level amputations.
b. Description of Related Art
For people who have experienced the partial loss of a limb such as the hand, prosthetic solutions are generally considered only after reconstructive surgical procedures have failed. In cases where a thumb is amputated or otherwise lost, an opposition post, cosmetic or not, is usually prescribed. With an intact thumb however, even if the metacarpals have been somewhat shortened, an orthotic device can be fitted for providing a post for the uninjured thumb to oppose. Alternatively a static cosmetic prosthesis may be fitted instead of the orthotic device. For cases that require a specialized fitting however, such fittings usually take the form of a custom made work prosthesis and/or a cosmetic glove for social occasions.
In many cases involving the loss of a thumb, transmetacarpal amputations of the fingers are often associated with loss of the thumb at or proximal to the metacarpophalangeal joint. In such cases, an active functional hand prosthesis is generally recommended only for cases where all digits (thumb and all four fingers) have been lost at a level equal or proximal to the metacarpophalangeal joint.
The range of prosthetic devices presently available for people with partial hand amputations can be broadly divided into the following three general categories; cosmetic prostheses, passive functional prostheses, and active functional prostheses. Active functional prostheses may be further subdivided into body-powered and externally-powered devices.
Cosmetic prostheses for persons with partial hand amputations typically consist of a handlike cosmetic glove, generally with the missing fingers filled with a urethane foam with wire reinforcements running through them. For persons whose loss is not severe enough to merit a full glove, individual cosmetic digits (i.e. fingers/thumbs) may be worn. A lack of adequate retention is however sometimes an issue with such individual prostheses for which adhesives and adhesive tape are often used for retention. Traditionally, cosmetic gloves are made of polyvinyl-chloride (PVC), while silicone is the material of choice for “high-end” cosmetic coverings.
Passive functional prostheses, also commonly known as task specific prostheses, are prosthetic devices donned when needed and thereafter removed once the task for which they are designed has been completed. The majority of such devices are work prostheses that bare little or no resemblance to the natural hand. Examples of such devices include perhaps a simple post for providing opposition, or a device incorporating specialized features to aid in the performance of specific occupations.
Lastly, body-powered prosthetic devices are generally functional prostheses that use some of the body's motion for providing the excursion necessary to actuate opening and closing of the device. Such body-powered prostheses for people with partial-hand amputations fall, for the most part, into one of two groups, those devices that are powered by biscapular abduction and/or gleno-humeral flexion, and those devices that are powered by flexion or extension of the wrist.
For a person with bilateral partial-hand amputations, a shoulder-driven device is often the system of choice, the control mechanism for which consists of a figure-of-nine harness that fits about the shoulders and a cable that runs from the harness to an appropriate terminal device. The most common terminal devices take the form of a hook. The disadvantages of such devices include poor cosmesis and the requirement for a harness and control cable to be worn, even though these devices do preserve the residual motion of the wrist.
One such device, known as the “Handy Hook,” uses a hook attached to the palmar surface of a partial hand prosthesis socket. Once attached through the use of a wrist attachment, the combination of the hook and wrist become one of the most functional prosthetic fittings, since wrist motion was preserved to assist the prehension process. One of the main disadvantages associated with this device however was that an individual had to wear a harness.
Another such system, known as the “Robin Aids Hand,” attempted to provide both function and cosmesis for body-powered partial hand fittings, and consisted of mechanical fingers that had interchangeable components which were mounted on a very short frame. This device was a voluntary opening device that could be fitted on patients who had either their thumb intact, small and ring fingers remaining, or no digits remaining. The thumb was an independent two-position unit, which allowed a larger grasp for items such as a glass or bottle after passive pre-positioning. As with the aforementioned “Handy Hook,” the “Robin Aids Hand” also required a harness and additionally a control cable. This device was also inefficient in operation and cosmetically displeasing. Whereas technological improvements have led to the development of more advanced body-powered prosthetic devices, such devices nevertheless restrict the work envelope of the amputee since tension in the control cable is required to actuate the terminal device. Since tension is not possible in all positions, the most common problem with such shoulder harness devices is that they cannot be stretched above the head, in which case the control cable becomes slack and sufficient tension cannot be developed to actuate the device. Another awkward position for such devices is in bending over to pick an object from the floor, in which case the control cable becomes too taut and cannot be slackened off to allow the terminal device to be driven.
For the second type of body-powered partial-hand prostheses, also referred to as wrist flexion and extension devices, such devices are functional cosmetic hands that operate in a manner similar to a tenodesis type hand orthosis (i.e. “tenodesis action” is a method by which prehension of the index finger, middle finger, and thumb is achieved through active wrist extension). These devices essentially operate by means of a linking mechanism which translates wrist extension into finger pinch and wrist flexion into finger opening. The user can thus determine where the fingers are and the forces involved through the wrist. This method is used extensively in Europe in the fitting of persons with partial hand amputation.
These aforementioned body-powered devices include drawbacks in that any remaining motion of the wrist that would normally be used to position a prehensor in space must be restricted to one degree-of-freedom (extension/flexion of the wrist) in order to drive the opening and closing of the terminal device. Additionally, while existing body-powered devices in general are aesthetically pleasing when static, their operation requires unnatural motions.
With regard to externally-powered prosthetic devices, such devices include for example externally-powered partial hand prosthesis for trans-carpal amputations. Such existing devices however are not suitable for amputations at the meta-carpophlangeal level, and do not preserve any residual motion of the wrist due to their size. An existing externally powered device for persons with partial hand amputations uses a single transverse motor in the line of the knuckles to open and close finger armatures over which a cosmetic glove is pulled. A drawback with this device is that the drive train is unable to handle high torques. Moreover, since this device does not drive a thumb, it is of limited use due to its limited pinch force and limited width-of-opening. Another existing externally-powered alternative for persons with partial hand amputations is to use a conventional powered hand and shorten its length. Such shortening enables the use of such devices for short trans-carpal cases. Such device however have limitations in that when shortened, the devices are ill suited for use in transmetacarpal cases.
Various conventional prosthesis mechanisms, some of which have been discussed above, are known and disclosed, for example, in U.S. Pat. No. 5,222,986 to Wright, U.S. Pat. No. 5,888,246 to Gow, U.S. Pat. No. 4,021,866 to Wasko, U.S. Pat. No. 4,291,421 to Massey et al., U.S. Pat. No. 4,258,441 to Bell, U.S. Pat. No. 4,990,162 to Le Blanc et al., U.S. Pat. No. 5,116,386 to Scribner and U.S. Pat. No. 4,865,929 to Eck, the respective disclosures of which are incorporated herein by reference. As discussed above, none of the prosthesis mechanisms disclosed in the above-identified U.S. patents overcome the aforementioned exemplary drawbacks in currently available prosthesis mechanisms.
Accordingly, there remains a need for a hand prosthesis, which may be adequately retained on a user's hand, is cosmetically acceptable and pleasing, does not require a harness to be worn around the shoulders, is suitable for amputations at the meta-carpophlangeal level, and preserves residual wrist motion. There also remains a need for a hand prosthesis which is robust in design, efficient to operate, simple to assemble and disassemble, and which is economically feasible to manufacture.